Wave Turbulence in Shallow Water Models

Wave turbulence in shallow water models P. Clark di Leoni, P. J. Cobelli, and P. D. Mininni Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires and IFIBA, CONICET, Cuidad Universitaria, Buenos Aires 1428, Argentina (Dated: September 3, 2021) We study wave turbulence in shallow water flows in numerical simulations using two different approximations: the shallow water model, and the Boussinesq model with weak dispersion. The equations for both models were solved using periodic grids with up to 20482 points. In all simulations, the Froude number varies between 0.015 and 0.05, while the Reynolds number and level of dispersion are varied in a broader range to span different regimes. In all cases, most of the energy in the system remains in the waves, even after integrating the system for very long times. For shallow flows, non- − linear waves are non-dispersive and the spectrum of potential energy is compatible with ∼ k 2 scaling. For deeper (Boussinesq) flows, the non-linear dispersion relation as directly measured from the wave and frequency spectrum (calculated independently) shows signatures of dispersion, and the − spectrum of potential energy is compatible with predictions of weak turbulence theory, ∼ k 4/3. In this latter case, the non-linear dispersion relation differs from the linear one and has two branches, which we explain with a simple qualitative argument. Finally, we study probability density functions of the surface height and find that in all cases the distributions are asymmetric. The probability density function can be approximated by a skewed normal distribution as well as by a Tayfun distribution. I. INTRODUCTION cies. In some of these cases the compatibility is limited to the spectrum of certain fields (see, e.g., [25]), or to specific configurations used to generate the excitations. Turbulence and non-linear wave interactions in the Moreover, for many systems it is also not clear whether ocean surface are related to important processes in at- the weak turbulence approximation holds for all times, as mospheric sciences and oceanography, such as the ex- the solutions are not homogeneous in wavenumber space change of energy between the atmosphere and the ocean and at sufficiently small scales eddies may become faster [1, 2]. This exchange, in turn, plays an important role than waves resulting in strong turbulence [26]. in the dynamics of the planetary and oceanic boundary One of the most important applications of weak tur- layers, with consequences on the transport and mixing of bulence is in surface gravity waves. In oceanography, momentum, CO , and heat [3]. The incorrect modeling 2 the Phillips’ spectrum [27], derived using dimensional of these phenomena affects climate evolution predictions arguments from strong turbulence and considering the [4, 5]. Ocean surface waves are also of interest in the coupling between waves, was long considered to be cor- search of renewable energies [6]. rect. However, different observational and experimental There are several ocean surface models which provide data [28, 29], as well as numerical simulations [30], sug- an excellent framework for studying weak turbulence the- gest that the actual spectrum is closer to that of weak ory [7–10]. This theory was developed to describe the turbulence. In fact, Phillips himself suggested that his out-of-equilibrium behavior of systems of dispersive and spectrum may not be valid in the ocean [31]. Nonethe- weakly non-linear waves (see, e.g., [11, 12]). Unlike the- less, a scaling compatible with Phillips’ spectrum is still ories of strong turbulence, for waves and under the as- observed in numerical simulations [32] when the forcing sumption of weak nonlinearities, the equations for two- is strong. This suggests that while weak turbulence pro- point correlations can be closed and exact solutions with vides an elegant theoretical way to study wave turbulence constant flux can be found. Besides this assumption, it in the ocean, more considerations are necessary to appro- arXiv:1309.1744v5 [physics.flu-dyn] 13 Jun 2014 is also assumed that wave fields are homogeneous, and priately describe the diversity of regimes found in these that free waves are uncorrelated. flows [11]. Weak turbulence theory has been applied to capil- Most of the work done in ocean surface waves under lary and gravito-capillary waves [10], vibrations on a the weak turbulence approximation concerns deep water plate [13], rotating flows [14], and magnetohydrodynamic flows. But the theory can also be applied to the shallow waves [12, 15, 16]. For some of these systems, the pre- water case, i.e., for gravity waves whose wavelengths are dictions of the theory are compatible with results ob- large compared to the height of the fluid column at rest tained from experiments or from numerical simulations. (see [33, 34]). In this case, the theory leads to the predic- For example, see Refs. [17–19] for capillary waves, [20] for tion that the energy spectrum follows a k−4/3 behav- gravitocapillary waves, [21–23] for vibrations on a plate, ior. Behavior compatible with this prediction∼ was found and [24, 25] for magnetohydrodynamic waves. Although both experimentally and observationally [35–37]. It was agreement has been found between theory, numerical also found that an inertial range with a k−2 depen- simulations and experiments, there are also discrepan- dency can develop in the shallower regions∼ of the fluid. 2 The comparison between different shallow water models, with different degree of shallowness (and of dispersion) is therefore of interest, e.g., for the study of waves in basins with inhomogeneous depth. In the shallow water regime there are several models that can be considered to describe the ocean surface. There is the linear theory (see, e.g., [38]) which can pre- dict the dispersion relation of small amplitude waves, but which is insufficient to study turbulence. There are also non-linear theories, such as the shallow water model [39] FIG. 1. The shallow water geometry considered in the sim- for non-dispersive waves, as well as the Boussinesq model ulations: x and y are the horizontal coordinates, and z is [40] for weakly dispersive waves which is the one used in the vertical coordinate. The surface height is h, with h0 the some of the most recent works on the subject [34]. While height of the fluid column at rest. The fluid surface is at pressure p0. L is a characteristic horizontal length (assumed to the former non-linear model is valid in the strict shal- be much larger than h0). Gravity acts on the −zˆ direction low water limit, the latter can be used in cases in which and its acceleration has a value of g. the wavelengths are closer to (albeit still larger than) the depth of the basin. In the present work, we study turbulent solutions of branch corresponding to non-dispersive waves, and an- the shallow water model and of the Boussinesq equations other corresponding to dispersive waves. We interpret using direct numerical simulations. Previous numerical this as the result of short wavelength waves seeing an ef- studies considered the Hamiltonian equations for a po- fectively deeper flow resulting from the interaction with tential flow with a truncated non-linear term, or the ki- waves with very long wavelength. (e) The probability netic equations resulting from weak turbulence theory at density function of surface height can be approximated moderate spatial resolution [32, 41] (with the notable ex- by both skewed normal and Tayfun distributions. In the ception of [42]). Here, we solve the primitive equations, latter case, the parameters of the distribution are com- without truncating the non-linear terms, potentially al- patible with those previously found in observations and lowing for the development of vortical motions and of experiments [37]. strong interactions between waves, and with spatial res- olutions up to 20482 grid points. The paper is organized as follows. In section II we introduce both models, show the assumptions made in II. THE SHALLOW WATER AND BOUSSINESQ order to obtain them, derive their energy balance equa- EQUATIONS tions, and briefly discuss the predictions obtained in the framework of weak turbulence theory. In section III we Let us consider a volume of an incompressible fluid describe the numerical methods employed and the simu- with uniform and constant (in time) density, with its lations. Then, in section IV we introduce several dimen- bottom surface in contact with a flat and rigid bound- sionless numbers defined to characterize the flows, and ary, and free surface at pressure p0. A sketch illustrating present the numerical analysis and results. We present the configuration is shown in Fig. 1; x and y are the hori- wavenumber energy spectra and fluxes, time resolved zontal coordinates, z is the vertical one, h is the height of spectra (as a function of the wavenumber and frequency), the fluid column (i.e., the z value at the free surface), h0 frequency spectra, and probability density functions of is the height of the fluid column at rest, L is a character- the fluid free surface height. Finally, in section V we istic horizontal length, gravity acts on the zˆ direction − present the conclusions. The most important results are: and its value is given by g. It is assumed that L h0 as (a) As in previous experimental and observational studies we are interested in shallow flows. ≫ [35, 36] we find now in simulations that different regimes In the inviscid case, both the Euler equation and the arise depending on the fluid depth and the degree of non- incompressibility condition hold in the fluid body, linearity of the system. (b) We obtain a power spectrum of the surface height compatible (within statistical uncer- ∂v 1 tainties) with k−2 in the shallow water (non-dispersive) + (v ∇)v = ∇p gz,ˆ (1) case, and one∼ compatible with a k−4/3 spectrum as the ∂t · −ρ − fluid depth is increased using the∼ Boussinesq (weakly dis- ∇ v =0.

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